Crystal-controlled digital clock

ABSTRACT

A clock with coaxial digit wheels for one-minute, ten-minute and hour indications has a drive motor intermittently energized by an electronic switch constituting the final stage of a 20-stage binary frequency divider which is stepped, at a frequency of 220/60 Hz, by a crystal-controlled oscillator. The latter consists essentially of a dual-FET inverter of MOS type connected across a quartz crystal, similar inverters being used for the several stages of the frequency divider. A holding button or key may be manually operated to apply a zero-setting potential to all the stages for arresting the clock until the actual time matches its reading. The motor, with associated homing contacts, and its battery-operated driving circuit may be mounted as a detachable unit on the clock housing.

United States Patent 1191 Gortz et al.

CRYSTAL-CONTROLLED DlGlTAlL ClLOClli Inventors: Bernd Gortz, Erlangen near Nurnberg; Wolfgang Fehrenbacher, St. Georgen, both of Germany Kundo Kieninger & Uherlell, St. Georgen, Germany Filed: Mar. 16, 1972 Appl. No.: 235,196

[73] Assignee:

58/3179, 125 C, 50 R; 318/306, 330; ZOO/61.39

[451 titt is, 1974 3,603,073 9/1971 Tcrada et al 58/50 R Primary Examiner-Richard B. Wilkinson Assistant ExaminerU. Weldon Attorney, Agent, or Firm-Karl F. Ross; Herbert [5 7 ABSTRACT A clock with coaxial digit wheels for one-minute, tenminute and hour indications has a drive motor intermittently energized by an electronic switch constituting the final stage of a 20-stage binary frequency divider which is stepped, at a frequency of 2 /60 112, by a crystal-controlled oscillator. The latter consists essentially of a dual-PET inverter of MOS type connected across a quartz crystal, similar inverters being used for the several stages of the frequency divider. A holding button or key may be manually operated to apply a zero-setting potential to all the stages for ar resting the clock until the actual time matches its reading. The motor, with associated homing contacts,

[56] References Cited and its battery-operated driving circuit may be UNITED STATES PATENTS mounted as a detachable unit on the clock housing.

3,590,571 7/ 1971 Niznik 58/23 R 23 Claims, 12 Drawing Figures n, NW 3+ egg/Ira: 6 sear? tater W L Ca: 2 4 2 I I :E I filling -19) c7 SMEYBUFG PATENTEUJUHBW 3.823.546

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1 CRYSTAL-CONTROLLED DIGITAL CLOCK Our present invention relates to a digital clock of the crystaLcontrolled type wherein two or more digit wheels are operatively interconnected, advantageously in coaxial relationship, to display time indications of different denominational orders in a window of a housing such as the units and tens digits of the minutes and corresponding numerical values for the hours.

In commonly owned U.S. patent application Ser. No. 101,974 filed 28 September 1970 by Gunter Hummel and Erich Scheer, now U.S. Pat. No. 3,686,879, there has been disclosed an electrically driven clock of this type wherein a one-minute wheel and an hour wheel are mechanically coupled with respective contactcarrying disks mounted coaxially therewith and cooperating with associated selector drums which are settable to actuate an alarm in a chosen time position of the digit wheels.

In another commonly owned application, Ser. No. 151,877 filed June 197i by one of us (Wolfgang lFehrenbacher) now U.S. Pat. No. 3,685,280, a further improvement in such as timepiece has been described wherein the digit wheels are coupledby coacting lateral projections enabling the entrainment of a higherranking wheel by a lower-ranking one.

in such timepieces, therefore, only the lowestranking digit wheel needs to be stepped at suitable intervals (e.g. of one minute) in order to display always the correct time of day with a degree of resolution determined by the stepping cadence; the same principles apply of course if the time indications are to be extended, e.g. with the aid of additional digit wheels, to cover larger periods such as days or weeks.

Obviously, the degree of accuracy of such a timepiece, depends upon the precision with which a predetermined steppingpulse cadence can be maintained. The use of mechanical synchronizing means such as a balance Wheel, even when coupled with an inherently stable pulse generator such as a crystal-controlled oscillator, is not entirely satisfactory since the intermittently advancing clockwork subjects the synchronizing mechanism to shocks which may throw the system out of step.

Also, conventional devices of this character are relatively bulky and difficult to operate from a low-voltage power supply such as a flashlight battery or dry cell.

The general object of our invention is to provide an improved timepiece of the aforedescribed type, especially a portable clock, which avoids the drawbacks set forth and operates reliably over extended periods with no maintenance other than a possible replacement of batteries.

A more particular object is to provide a steppingpulse generator of low power consumption for a miniature electric motor driving the lowest-ranking digit wheel of such a timepiece. 7

It is also an object of this invention to provide a highly compact modular construction for a driving unit associated with such a timepiece.

In accordance with an important feature of our invention, the drive motor of the timepiece is periodically stepped by the output of a pulse generator which includes a crystal-controlled oscillator and a frequency divider energized thereby, this divider converting the crystal-stabilized oscillations into a succession of stepping Pulses each occurring during a unit of time (e.g.

one minute) of the lowest denominational order to be displayed by the digit wheels; each stepping pulse, lasting for only a fraction of such a unit of time, energizes the motor long enough to drive a rotary member, positively coupled with its rotor, through part of a revolution whereupon a homing circuit controlled by that member keeps the motor operating until the revolution is completed. Such a homing circuit may include a pair of cam-operated contacts closing the motor circuit near the trailing edge of the stepping pulse forming part of a square wave emitted by a flip-flop which constitutes the final stage of the frequency divider. The exact home position of the rotary member is not critical if, pursuant to a further feature of our invention, this member forms part of a Geneva-type transmission drivingly engaging a co-operating element over only a fraction of its own revolution. Such a transmission, advantageously, includes a sector gear as the driving member and a full gear as the driven element, a concentric dwell on the sector gear serving to arrest the driven gear in the position last reached upon its entrainment by the toothed portion of the driving gear.

in accordance with a further feature of our invention, the miniature drive motor together with its homing circuit forms part of a unit which is detachably mounted on the digitwheel housing, the associated pulse generator being carried on the housing as an integrated module and being electrically connected to the motor circuit through flexible conductors which are long enough to enable physical detachment of the driving unit from the housing.

ln this way, either component may be made accessible from all sides for the purpose of inspection or repair.

A further feature of our invention resides'in the provision of a manually operable switch which is preferably carried on the detachable unit and serves for the temporary deactivation of the stepping-pulse generator to resynchronize the clock by arresting the digit wheels in the position in which they happen to be or into which they are manually moved; at the instant when this wheel position corresponds to the actual time, e.g. as indicated by a radio signal, the user restores the switch to restart the clockwork.

A particularly efficient oscillator for our improved motor drive comprises a pair of complementary field effect transistors with a common gate lead and a common drain lead connected to opposite terminals of a frequency-stabilizing crystal, hereinafter referred to simply as quartz. The two complementary F.E.T.s, which may be of the monolithic type known in the art as COS/MOS, constitute an inverter whose output is fed back to the quartz to establish a voltage difference thereacross. With the gate lead biased at an intermediate potential by two identical resistors extending from that lead to respective source electrodes of the two F.E.T. s, a pair of storage capacitors inserted between the two quartz terminals and a point of fixed potential (such as one of the two source electrodes) will alternately acquire positive and negative charges in the rhythm of the natural frequency of the quartz as modified by a trimmer condenser in series therewith. Such an oscillator requires no lumped inductances and can, therefore, be easily realized by integrated-circuit technique.

Pairs of similar inverters may be included in respective stages of the binary frequency divider following the oscillator. All these components can, therefore, be readily accommodated on a printed strip mounted on the digit-wheel housing, preferably on a rear wall of that housing parallel to the wheel axis.

A miniature drive motor, stepped by such a frequency divider through an amplifier advantageously including two cascaded complementary transistors, can be operated with a conventional battery of, say, 1.5 V; the slightly higher voltage requirements of the oscillator, the frequency divider and the input circuits of the cascaded transistors can be satisfied by a separate battery of about 6-9 V. Since the energy consumption of the electronic components is small, excess power will be available to operate ancillary equipment such as, for example, an alarm circuit.

The above and other features of our invention will be described in detail hereinafter with reference to the accompanying drawing in which:

FIG. 1 is a front elevational view (with parts broken away) of a timepiece embodying our invention;

FIG. 2 is a bottom view of the assembly of FIG. 1;

FIG. 3 is an exploded perspective view of the digital clockwork of the timepiece shown in FIGS. 1 and 2;

FIG. 4 is a circuit diagram of the motor drive for the clockwork;

FIG. 5 shows details of an oscillator included in the diagram of FIG. 4;

FIG. 6 is an end view of a modified driving unit for a timepiece according to our invention;

FIG. 7 is a side view of the unit illustrated in FIG. 6;

FIG. 8 is a top view of the same unit;

FIG. 9 is a detail view of part of the power train of the drive unit as seen from above, drawn to a scale larger than that of FIG. 8;

FIG. 9a is a face view of a homing cam also seen in FIG. 9;

FIG. 9b is a face view of a driving gear seen in FIG. 9 and of a coacting driven gear; and

FIG. 10 is a top view, drawn to a smaller scale than FIG. 8, of an entire timepiece provided with the unit of FIGS. 6 8.

In FIGS. 1 and 2 we have shown parts of the housing of an electric digital clock including a bracket l with two upstanding arms 2, 3. This housing has a window, not shown, for displaying the time of day (here 16:36 hours or 4:36 PM) in the form of digits carried by three coaxial wheels 4, 5 and 6. Digit wheel 4, bearing the numerals 0 to 9, and digit wheel 5, bearing the numerals O to 5, show the minutes; digit wheel 6 displays the hours with the aid of eight peripherally mounted prismatic bodies of which three, designated 8, 9 and 10, are visible in FIG. 1. These bodies are in the form of regular three-sided pyramids provided, as shown in the above-identified application and patent by Hummel et al, with star wheels each carrying angularly spaced pins engageable by a fixed stud in a stationary end cap 11 secured to arm 2. Reference in this connection may also be made to commonly owned US. Pat. No. 3,616,642. Thus, during any revolution of wheel 6, any digit carrier 8, 9, 10 etc. is tripped on passing through its nadir position and rotated through 120 to exhibit a new face to the observer; these faces, 24 in all, carry the numerical designations from 00 to 23. Naturally, the 24 hour readings could also be designated 1 AM to 12 AM and 1 PM to 12 PM. Each digit carrier may be held in its position relative to wheel 6, for a full revolution of that wheel, by friction or with the aid of indexing means not shown.

Bracket 1 supports, via posts 13 and screws 14, a similar, smaller bracket 15 having upstanding arms 16 and 17. The arm 3 of bracket 1 is broadened to serve as a mounting plate for a motor 22 and other parts of the stepping drive, the motor having a base 31 supported on plate 3 by posts 21.

Digit wheels 4, 5 and 6 are rotatably journaled on three fixed shafts 19, 18 and 7, respectively. Shaft 7 extends between uprights 2 and 16; shaft 18 spans the arms 16 and 17 of bracket 15 which lie within the concave sides of the two hollow wheels 6 and 4, confronting the intermediate wheel 5, while shaft 19 is secured to uprights 17 and 3 in line with shaft 7. The two aligned shafts 7 and 19 are offset from shaft 18 by a distance equal to the difference between the radii of the two larger wheels 4, 6 (which are of like diameter) and of the smaller wheel 5 whereby the centrally disposed faces of the polygonal peripheries of the wheels 4, 5 and 6 as viewed in FIG. 1 (constituted in the case of wheel 6 by an exposed prism face such as that of prism 9) are coplanar in their display position. Since the number of these faces of wheels 4 and 5 are ten and six, respectively, the ratio of their radii is approximately 5:3; this affords sufficient clearance to accommodate the bight portion of bracket 15 adjacent wheel 5 within the confines of wheels 4 and 6.

Wheel 5 is also laterally recessed, as seen in FIG. 3, so as to have two overhanging rims 111, 136 corresponding to a rim on the concave side of wheel 6 and a similar rim on the confronting face of wheel 4. The spaces framed by rim 100 of wheel 6 and by the corresponding rim of wheel 4 serve-to accommodate indexing means constituted, in the case of wheel 6, by a ratchet 101 normally engaged by a pawl 103 which is swingable about a fulcrum 104 and has its free end 105 stressed by a tension spring 107 anchored at 106 to the body 102 of the digit wheel. The analogous indexing mechanisms for wheels 4 and 5 have not been illustrated. The free end of an escapement lever on the remote face of wheel 5 is integral with an axially extending projection or dog 110 which is normally held by a loading spring against the inner peripheral surface of rim 111 so as to lie in the path of a set of cooperating projections 109 of wheel 6 formed as internal ribs on the rim 100 thereof. Similar projections 130, in the shape of prismatic studs, extend axially from the inner periphery of rim 136 of wheel 5 into the concave side of wheel 4 for coupling engagement with a single dog I31 fixed within that side to the rim of the latter wheel. In conformity with the number of active faces of wheels 5 and 6, ratchet 101 and the corresponding ratchet of wheel 5 have eight and six teeth, respectively; similarly, there are eight projections 109 on wheel 6 and six projections on the wheel 5.

Thus, in view of the relative eccentricity of the orbits of projections 130, 131 and 109, 110, wheel 5 is stepped once per revolution of wheel 4 through an angle of 60 whereas the wheel 6 is advanced once per revolution of wheel 5 through an angle of 45. The stepping drive for wheel 4 includes, besides the synchronous motor 22, a gear train 23-30; if this drive comprises a stage with positive immobilization of the load, such as a Geneva motion or the equivalent transmission described hereinafter with reference to FIG. 9b, the

- aforementioned indexing mechanism for digit wheel 4 may be omitted.

The presence of the lever carrying the dog 1110 has the effect of an overrunning clutch by permitting the hour wheel 6 to overtake the ten-minute wheel 5 on being manually rotated (clockwise as viewed in FIG. 3) to change the hour indication, with the projections 1109 camming aside the projection 1110 by bearing upon a beveled outer face thereof if the latter projection happens to lie between the two foremost ribs 109', 1109" of wheel 6 as illustrated in FIG. 3, i.e., if wheel 5 occupies the 50-minute position in which its next step would advance the wheel 6 from, say, the ll-oclock to the l2-oclock position.

Thus, regardless of the relative position of the several coupling projections, the positions of the two minute wheels 4 and 5 will remain unaffected by any adjustment of hour wheel 6 so as to eliminate the need for any subsequent readjustment of the minute indications. On the other hand, a manual advance of l-minute wheel 4 may also step the 10-minute wheel 5 and possibly the hour wheel 6, in the same way as does the automatic drive described below.

In view of the play afforded by the indexing means and their counterparts on wheel 5, the range of positive entrainment of wheel 5 by wheel 4 and of wheel 6 by wheel 5 may be somewhat less than 60 and 45, respectively, inasmuch as the force of loading spring 107 and its counterpart will in such case complete the rotation of the follower wheel through the desired angle.

Digit wheels 4 and 6 are shown provided with toothed peripheries 64 and 65 facilitating the setting of the minute and hour indications, e.g. with the aid of nonillustrated pinions meshing therewith and projecting from the housing for manual actuation as described in the aforementioned US. Pat. No. 3,685,280.

Motor 22 has an operating circuit, more fully illustrated in FIG. 4, including a normally open homing switch 33 in the form of contact springs riveted at 36 in mutually insulated relationship to a stud 37 which projects from mounting plate 3. One of the contact springs forms a tongue 34 bearing upon a rotary cam disk 35 on the shaft of transmission gear 30. Cam 35, more fullydescribed below with reference to FlG. 9a, has a dwell which closes the contacts 33 whenever the motor 22 is briefly energized by the pulse generator of FIG. 4 to rotate the gear and its shaft through an arc of, say, a quarter of a circle. Closure of the homing circuit then maintains the motor operated until the cam has performed a full revolution. The step-down ratio of the gear train 23 30 is so chosen that such a revolution drives the wheel 4 through an arc of 36, corresponding to its peripheral subdivision by the 1- minute markings. The motor speed is advantageously so regulated that a revolution of cam 35 lasts somewhat less than a minute so that the motor will be restarted by the next stepping pulse after a rest period which may vary with the supply voltage and other external parameters.

Homing contacts 33 are representative of a variety of switches that can be used for this purpose, e.g. reed switches responsive to a pair of magnets of opposite polarity mounted with the requisite angular spacing on a disk replacing the cam 35.

Reference will now be made to FIG. 4 which shows the electronic components serving for the intermittent energization of motor 22. These components include an oscillator 40 consisting essentially of a quartz 0, an inverter $0, a pair of storage capacitors C C and a trimmer condenser C in series with the quartz Q. The oscillator 40, more fully illustrated in FIG. 5, is con nected across a supply battery of 9 V whose terminals are tied to a positive bus bar 411 and a negative bus bar (here grounded) 4-2; a current-limiting resistor R is inserted in bus bar 411. Between these bus bars we provide a voltage divider consisting of two substantially identical resistors R R whose junction, biased to an intermediate d-c potential of about 4.5 V, feeds a gate lead 43 of inverter SQ which is constituted by two complementarily symmetrical monolithic semiconductors COS/MOS, specifically field-effect transistors. Quartz Q and trimmer condenser C lie between the gate lead 43 and a common drain lead 44 of the two F.E.T.s which in FIG. 5 have been designated P and N according to their respective conductivity types. Capacitor c lies between negative bus bar 42 and the drain lead 44 connected to one terminal of quartz Q. Capacitor C lies between the same bus bar and the other quartz terminal (in series with condenser C being thus arranged in parallel with resistor R This oscillator can operate with high frequency stability even in a range as low as a few kHz.

In the absence of the feedback loop from drain lead 44 to quartz Q, the current paths through transistors P and N would be equally conductive. Because of the feedback, however, a slight unsymmetry in the voltage distribution is translated into a magnified potential difference across quartz Q and condenser C whereby the quartz 0 starts oscillating and alternately charges the capacitors C and C at a frequency here assumed to equal 17.47627 kHz. This frequency is stepped down by two cascaded multistage frequency dividers ST and 5T of 14 and 6 binary stages, respectively, there being thus a total of 20 stages with a resulting step-down ratio of 1:2 This corresponds to an output frequency of 1/60 Hz, or one pulse per minute, representing the cadence of a square wave generated on a lead 46 by the final stage of divider 8T Each divider stage advantageously is a flip-flop with a pair of inverters of the COS/MOS F.E.T. type, similar to circuit SQ, cross-connected in the conventional manner for bistable operation. Each of the two inverters of each stage has its common drain connected to a respective lead 144', 1144" and 244 (corresponding lead 44 of FIG. 5), 244". The leads 1144' and 244 of all stages are permanently grounded at bus bar 42 whereas the companion leads 1144" and 244 are connected to ground through a manually operable switch 45 and further connected through a large resistor R, to positive bus bar 411. With switch 45 in its normal closed position, the inverters with drains tied to conductors 144" and 244" operate in the same manner as the inverter SQ of FIG. 5 so that the respective flip-flop counts the incoming pulses or voltage surges from the preceding stage or from oscillator 40. If, however, switch 45 is opened, the drains of the affected inverters go positive so that all the stages are reset to and maintained in what may be considered their 0" condition. This interrupts the pulse train on output lead 46 so that motor 22 is no longer stepped.

The connection between lead 46 and motor 22 includes a coupling condenser C in the input of a first amplifier stage constituted by a PM? transistor T the emitter of this transistor being connected directly to bus bar 41 and to its base through a voltage divider R R whose junction is tied to condenser C The collector of transistor T feeds, through a resistor R the base of an NPN transistor T constituting a second amplifier stage; the emitter of the latter transistor is grounded at bus bar 42 whereas its collector is connected through the field winding of motor 22 to an ancillary bus bar 47 which is maintained by another battery at a potential of +1.5 V. Closure of contacts 33 by the cam 35, upon energization of motor 22 by a pulse traversing the condenser C shunts the emitter-collector circuit of transistor T so as to keep the motor 22 directly energized from the 1.5-V power supply until contacts 33 reopen. If desired, a resistive and capacitive braking circuit for motor 22 may be completed by the switch 33 upon the reopening of the homing circuit to prevent the motor from overshooting the home position of cam 35.

In FIGS. 6 10 we have shown a demountable unit 50 containing the motor 22 with its housing, cam 35 and part of the transmission by which this motor is coupled with the digit wheels 4 6.

A mounting plate 200, adjoining the plate 3 of the digit-wheel housing 236, is detachably secured to the latter plate by elastic studs 222 held in position thereon by screws 224, the studs 222 passing through complementary apertures 223 of plate 200 in frictional engagement therewith. Bolts 201, provided with nuts 202, spacedly support an ancillary plate 203 on plate 200. A bracket 204, secured to plate 200 by screws 205 (only one shown), carries the drive motor 22 whose vertical shaft 206, lying skew to the axis of wheels 4-6, has keyed to it a worm 23 forming part of the transmission 23 30 of FIGS. 1 and 2. Other elements of this transmission include a worm gear 24 in mesh with worm 23, carried on a shaft 207 which is joumaled in the plates 200, 203 so as to extend in the axial direction of digit wheels 4 6; a pinion 25 keyed to the same shaft; a spur gear 26 on a stub shaft 208 driven by pinion 25; and another spur gear 27 whose shaft 210 also spans the plates 200 and 203. Shaft 210 further carries the cam disk 35 as well as a sector gear 30a, more clearly illustrated in FIGS. 9 and 9b, which intermittently drives another spur gear 30b also seen in FIG. 9b; the shaft 211 of gear 30b, journaled in plates 200 and 203, has an extension 212 beyond mounting plate 200 carrying a spur gear 213 which in the assembled condition meshes with another gear on the shaft 19 of digit wheel 4. A split ring 209 holds the gear 26 in position on stub shaft 208, behind two pairs of leaf springs 216, 219 which form part of the homing-switch assembly 33 and terminate in contact blobs 214, 217. The lower springs 216 merge into the tongue 34 which coacts with cam disk 35. A further bracket 225, riveted to plate 200, carries the manual switch 45 designated as a knob on a vertical post 49 which constitutes an ancillary supporting leg for the unit 50.

As illustrated in FIG. 9, sector gear 300 is rigid with spur gear 27 with which it may have been integrally molded from plastic material. The toothed portion 230 of this gear, extending over less than half its circumference, adjoins a cylindrical portion 226, centered on the axis of shaft 210, which in the home position engages two neighboring teeth of driven gear 30b so as to prevent rotation of the latter. Cylindrical gear portion 226 is axially extended toward cam 35 so as to complement a part-cylindrical hub portion 227 of the latter whereby the disk 35 is positively coupled with the integral assembly 27, 30a for joint rotation. Axial separation of hub portions 226 and 227 is prevented by a washer 48 held in position by a cotter pin not shown.

The outline of cam 35, as seen in FIG. 9a, includes a low dwell 241 extending over approximately a rising flank 243 leading therefrom to a high dwell 242 which spans about and a radial edge 240. Upon clockwise rotation of the cam as indicated by the arrow, tongue 34 is lifted onto dwell 242 to close the contacts 214, 217 well before the stepping pulse then energizing the motor 22 has terminated. Continuity of rotation is thus assured until the tongue 34 drops off the edge 240, thereby instantly opening the motor circuit and arresting the cam. Since the concentric portion 226 of driving gear 300 immobilizes the driven gear 30b at this stage of the cycle, the operation of the clockwork is not affected by minor departures of the cam 35 from its theoretical home position. The number of teeth of gear portion 230 depends, of course, on the transmission ratio of the coupling between shafts 212 and 19 which may include a ratchet drive to facilitate manual setting of wheel 4 with the gear 36b blocked. A single tooth may suffice in a limiting case.

The elements mounted on plate 200 are electrically connected to a printed strip 229, carrying the components 40, ST,, 8T T and T of FIG. 4, through flexible conductors 228 which enable the physical separation of unit 50 from the digit wheels 4 6 without detaching the strip 229 from the clock housing 236 on which it is mounted. In the illustrated embodiment, the strip 229 extends along the rear wall of the housing, parallel to the wheel axis, on the side remote from the window through which the digit wheels can be read. Upon such removal of the mechanical driving unit from the mounting plate 3, gear 213 is disengaged from its mate so that the unit can be electrically tested independently of the load normally driven thereby.

The battery or batteries used to power the motor circuit may be accommodated on the clock housing 236 or on the detachable unit 50.

We claim:

1. A time piece comprising:

a set of operatively interconnected digit wheels including a first wheel provided with time indications of a relatively low denominational order and a second wheel provided with time indications of a relatively high denominational order;

drive means including a rotary member, a motor positively connected with said member and a power source for said motor;

coupling means connecting said member with said first wheel;

a generator of stepping pulses for said motor energized by said power source, said generator including a crystal-controlled oscillator and a frequency divider energized by said oscillator to deliver to said motor a succession of stepping pulses each occurring during a unit of time of said low denominational order and lasting long enough to rotate said member through a fraction of a revolution; and

homing means coupled with said motor for maintaining same energized from said power source beyond the duration of said stepping pulse to drive said member through a full revolution within said unit of time, said coupling means translating said full revolution into an advance of said first wheel by one time indication.

2. A timepiece as defined in claim 1 wherein said unit of time is a minute and said frequency divider has 20 binary stages.

3. A timepiece as defined in claim ll wherein said member is a driving gear with a toothed portion extending over a fraction of a circle, said coupling means including a driven gear positioned for intermittent entrainment by said driving gear in off-normal rotary positions of the latter.

4. A timepiece as defined in claim 3 wherein said driving gear is provided with a concentric dwell adjoining said toothed portion for arresting said driven gear in the position last reached upon entrainment by said toothed portion.

5. A timepiece as defined in claim 3 wherein said homing means comprises a cam coaxially mounted with said driving gear for joint rotation therewith and contact means actuatable by said cam in an off-normal rotary position thereof.

b. A timepiece as defined in claim 5 wherein said driving gear and said cam are provided with complementary hub portions engaging each other for positive rotary coupling, further comprising a common shaft supporting said hub portions and removable stop means on said shaft preventing disengagement of said hub portions by axial separation.

7. A timepiece as defined in claim ll wherein said digit wheels are provided with a common housing, further comprising a unit detachably mounted on said housing and including said drive means together with said homing means and part of said coupling means.

8. A timepiece as defined in claim 7 wherein said generator includes integrated circuitry on a wall of said housing, said circuitry being connected to said drive means by flexible conductors long enough to enable physical detachment of said unit from said housing.

9. A timepiece as defined in claim 3 wherein said housing is provided with elastic supporting studs projecting therefrom in an axial direction of said digit wheels, said unit including a mounting plate with perforations frictionally accommodating said studs.

10. A timepiece as defined in claim 9 wherein said digit wheels are coaxially disposed in said housing, said circuitry comprising a printed strip lying parallel to the digit-wheel axis.

1111. A timepiece as defined in claim 9 wherein said mounting plate is provided with a bracket supporting said motor with its axis of rotation skew to said digitwheel axis.

l2. A timepiece as defined in claim Ill, further comprising a worm directly connected with said motor, a worm gear in mesh with said worm and a set of intermeshing spur gears transmitting the torque of said worm gear to said member, said worm gear and said spur gears being rotatable about axes parallel to said digit-wheel axis.

l3. A timepiece as defined in claim 7 wherein said unit includes a manually operable switch between said power source and said frequency divider for temporarily deactivating said drive means.

M. A timepiece as defined in claim ll, further comprising manually operable holding means for temporarily deactivating said frequency divider in a selected position of said digit wheels.

15. A timepiece as defined in claim 11 i wherein said frequency divider comprises a multiplicity of cascaded binary counting stages each provided with a resetting input, said holding means including a switch operable to apply a blocking potential to the resetting inputs of all said stages.

16. A timepiece as defined in claim 15 wherein said stages include monolithic field-effect transistors.

17. A timepiece as defined in claim 1 wherein said oscillator comprises a pair of complementary fieldetfect transistors with a common gate lead and a common drain lead, a frequency-stabilizing crystal between said leads, and a direct-current power supply connected between respective source electrodes of said transistors.

1%. A timepiece as defined in claim 17, further comprising a pair of identical biasing resistances connected between said gate lead and said source electrodes, a first storage capacitor connected between said gate lead and a point of fixed potential, another storage capacitor connected between said drain lead and said point, and a trimmer condenser between said leads in series with said crystal and said storage capacitors.

l9. A timepiece as defined in claim 11'! wherein said field-effect transistors are part of a monolithic inverter.

20. A timepiece as defined in claim 1 wherein said generator further includes amplifier means between said frequency divider and said motor, said amplifier means comprising a pair of cascaded complementary transistors with a capacitive input connection to the output of said frequency divider.

21. A timepiece as defined in claim Zfi wherein said generator is provided with a first direct-current power supply of relatively high voltage for said oscillator, said frequency divider and the input circuits of said cascaded transistors and with a second direct-current power supply of relatively low voltage for said motor and the output circuit of the second cascaded transistor.

22. A timepiece as defined in claim 2l wherein said first and second power supplies are batteries of substantially 6 to 9 volts and of substantially 1.5 volts, respectively.

23. A timepiece as defined in claim ll wherein said rotary member fonns part of a Geneva-type transmis- 

1. A time piece comprising: a set of operatively interconnected digit wheels including a first wheel provided with time indications of a relatively low denominational order and a second wheel provided with time indications of a relatively high denominational order; drive means including a rotary member, a motor positively connected with said member and a power source for said motor; coupling means connecting said member with said first wheel; a generator of stepping pulses for said motor energized by said power source, said generator including a crystal-controlled oscillator and a frequency divider energized by said oscillator to deliver to said motor a succession of stepping pulses each occurring during a unit of time of said low denominational order and lasting long enough to rotate said member through a fraction of a revolution; and homing means coupled with said motor for maintaining same energized from said power source beyond the duration of said stepping pulse to drive said member through a full revolution within said unit of time, said coupling means translating said full revolution into an advance of said first wheel by one time indication.
 2. A timepiece as defined in claim 1 wherein said unit of time is a minute and said frequency divider has 20 binary stages.
 3. A timepiece as defined in claim 1 wherein said member is a driving gear with a toothed portion extending over a fraction of a circle, said coupling means including a driven gear positioned for intermittent entrainment by said driving gear in off-normal rotary positions of the latter.
 4. A timepiece as defined in claim 3 wherein said driving gear is provided with a concentric dwell adjoining said toothed portion for arresting said driven gear in the position last reached upon entrainment by said toothed portion.
 5. A timepiece as defined in claim 3 wherein said homing means comprises a cam coaxially mounted with said driving gear for joint rotation therewith and contact means actuatable by said cam in an off-normal rotary position thereof.
 6. A timepiece as defined in claim 5 wherein said driving gear and said cam are provided with complementary hub portions engaging each other for positive rotary coupling, further comprising a common shaft supporting said hub portions and removable stop means on said shaft preventing disengagement of said hub portions by axial separation.
 7. A timepiece as defined in claim 1 wherein said digit wheels are provided with a common housing, further comprising a unit detachably mounted on said housing and including said drive means together with said homing means and part of said coupling means.
 8. A timepiece as defined in claim 7 wherein said generator includes integrated circuitry on a wall of said housing, said circuitry being connected to said drive means by flexible conductors long enough to enable physical detachment of said unit from said housing.
 9. A timepiece as defined in claim 8 wherein said housing is provided with elastic supporting studs projecting therefrom in an axial direction of said digit wheels, said unit including a mounting plate with perforations frictionally accommodating said studs.
 10. A timepiece as defined in claim 9 wherein said digit wheels are coaxially disposed in said housing, said circuitry comprising a printed strip lying parallel to the digit-wheel axis.
 11. A timepiece as defined in claim 9 wherein said mounting plate is provided with a bracket supportiNg said motor with its axis of rotation skew to said digit-wheel axis.
 12. A timepiece as defined in claim 11, further comprising a worm directly connected with said motor, a worm gear in mesh with said worm and a set of intermeshing spur gears transmitting the torque of said worm gear to said member, said worm gear and said spur gears being rotatable about axes parallel to said digit-wheel axis.
 13. A timepiece as defined in claim 7 wherein said unit includes a manually operable switch between said power source and said frequency divider for temporarily deactivating said drive means.
 14. A timepiece as defined in claim 1, further comprising manually operable holding means for temporarily deactivating said frequency divider in a selected position of said digit wheels.
 15. A timepiece as defined in claim 14 wherein said frequency divider comprises a multiplicity of cascaded binary counting stages each provided with a resetting input, said holding means including a switch operable to apply a blocking potential to the resetting inputs of all said stages.
 16. A timepiece as defined in claim 15 wherein said stages include monolithic field-effect transistors.
 17. A timepiece as defined in claim 1 wherein said oscillator comprises a pair of complementary field-effect transistors with a common gate lead and a common drain lead, a frequency-stabilizing crystal between said leads, and a direct-current power supply connected between respective source electrodes of said transistors.
 18. A timepiece as defined in claim 17, further comprising a pair of identical biasing resistances connected between said gate lead and said source electrodes, a first storage capacitor connected between said gate lead and a point of fixed potential, another storage capacitor connected between said drain lead and said point, and a trimmer condenser between said leads in series with said crystal and said storage capacitors.
 19. A timepiece as defined in claim 17 wherein said field-effect transistors are part of a monolithic inverter.
 20. A timepiece as defined in claim 1 wherein said generator further includes amplifier means between said frequency divider and said motor, said amplifier means comprising a pair of cascaded complementary transistors with a capacitive input connection to the output of said frequency divider.
 21. A timepiece as defined in claim 20 wherein said generator is provided with a first direct-current power supply of relatively high voltage for said oscillator, said frequency divider and the input circuits of said cascaded transistors and with a second direct-current power supply of relatively low voltage for said motor and the output circuit of the second cascaded transistor.
 22. A timepiece as defined in claim 21 wherein said first and second power supplies are batteries of substantially 6 to 9 volts and of substantially 1.5 volts, respectively.
 23. A timepiece as defined in claim 1 wherein said rotary member forms part of a Geneva-type transmission. 